In the field of microelectronic device manufacturing, devices may be subjected to a variety of testing to determine the existence of electrical defects. Although testing whether a device is “good” or “bad” is generally straightforward, difficulty lies in determining the location of any defects. Isolating the exact layer in which a defect is located may be important to determining the root cause of failures; e.g., failures may arise from problems in the manufacturing process, perhaps at a particular process stage, and/or equipment.
Currently, there are several methods for isolating defects. For example, time domain reflectometry has been used to isolate defects in a device. Time domain reflectometry is a method of transmitting an electrical pulse to a conductor. Defects (such as opens and shorts in a die or package) reflect the electrical pulse, and the reflected pulse is measured as a function of time. The reflected measurement is compared to a reference to determine the exact location of the defect. The problem with this method is that the electrical pulses have a relatively long rise time and thus the reflected waveform does not enable fine resolution of the location of a defect. This problem makes this method ineffective in identifying defects in complex packages and/or isolation if faults within layers or between a die and a package. Furthermore, this method usually involves destructive analysis by physically grinding away each layer of a device and testing after each layer is grinded away which may be time-consuming and clearly do not permit use of the device once destroyed.
Electro-optic sampling is a sampling method that has been enlisted to determine electric fields in various semiconductor devices. Electro-optic sampling can be considered a high-frequency sampling oscilloscope and thus has a benefit of being able to sample a device on a very short time interval. However, this method may not be able to determine an exact location of a defect in a device or in a device layer.